I'll purchase routine maintenance items 2-3 weeks before I'm ready to do the maintenance. There are a few things such as headlight, tail light and brake light bulbs that I keep on hand for each vehicle so I can change them immediately when they go out.
I also keep the old set of wipers in each vehicle when I change them out-I had one fall off in the winter years ago and learned my lesson.
There are a few key N tractor parts that I keep on hand-front mount coil, points, condenser, plugs, key switch, spare tire tube, etc. If one fails in the middle of winter and I need to plow snow I can have a new one installed and the tractor up and running again in a few minutes.
I don't stock up on oil, I just keep enough part containers on the shelf to top off anything that gets low before the next oil change. I keep a couple gallons of wiper fluid on hand so I can keep the vehicles topped off when the weather is sloppy.